This invention relates to an improved carbonaceous heat source and to the conversion of gaseous by-products, such as carbon monoxide, produced by the combustion of the carbonaceous heat source and methods to make such a heat source. The methods and heat source of this invention are particularly suitable for use in a smoking article such as that described in U.S. Pat. No. 4,991,606 commonly assigned herewith. The heat sources of this invention comprise a carbonaceous material and smaller amounts on a weight basis of a metal oxide. The heat sources of this invention have low ignition temperatures and high combustion temperatures. Upon combustion, the metal oxide component of the heat source chemically converts substantially all of the carbon monoxide to a benign substance.
According to this invention, carbon or a material that can be subsequently converted to carbon is pre-formed into a shape. The pre-formed carbonaceous material is treated with a sol which comprises a metal oxide precursor that subsequently can be converted to a metal oxide. This results in the deposition of the metal oxide precursor into the pre-formed carbonaceous material. The pre-formed carbonaceous material is further treated to convert the deposited metal oxide precursor into metal oxide, thus producing a heat source with a low ignition temperature, durability for handling, a large heat release during combustion, and low emission of carbon monoxide during combustion that can be produced by a simplified method.
There have been previous attempts to provide a heat source for a smoking article. While providing a heat source, these attempts have not produced a heat source having all of the advantages of the present invention.
For example, Siegel U.S. Pat. No. 2,907,686 describes a charcoal rod coated with a concentrated sugar solution which forms an impervious layer during burning. It was thought that this layer would contain gases formed during smoking and concentrate the heat thus formed.
Ellis et al. U.S. Pat. No. 3,258,015 and Ellis et al. U.S. Pat. No. 3,356,094 describe a smoking device comprising a nicotine source and a tobacco heat source.
Boyd et al. U.S. Pat. No. 3,943,941 describes a tobacco substitute which consists of a fuel and at least one volatile substance impregnating the fuel. The fuel consists essentially of combustible, flexible and self-coherent fibers made of a carbonaceous material containing at least 80% carbon by weight. The carbon is the product of the controlled pyrolysis of a cellulose-based fiber containing only carbon, hydrogen and oxygen.
Bolt et al. U.S. Pat. No. 4,340,072 describes an annular fuel rod extruded or molded from tobacco, a tobacco substitute, a mixture of tobacco substitute and carbon, other combustible materials such as wood pulp, straw and heat-treated cellulose or a sodium carboxymethylcellulose (SCMC) and carbon mixture.
Shelar et al. U.S. Pat. No. 4,708,151 describes a pipe with replaceable cartridge having a carbonaceous fuel source. The fuel source comprises at least 60-70% carbon, and most preferably 80% or more carbon, and is made by pyrolysis or carbonization of cellulosic materials such as wood, cotton, rayon, tobacco, coconut, paper and the like.
Banerjee et al. U.S. Pat. No. 4,714,082 describes a combustible fuel element having a density greater than 0.5 gm/cc. The fuel element consists of comminuted or reconstituted tobacco and/or a tobacco substitute, and preferably contains 20%-40% by weight of carbon.
Published European patent application 0 117 355 by Hearn et al. describes a carbon heat source formed from pyrolized tobacco or other carbonaceous material such as peanut shells, coffee bean shells, paper, cardboard, bamboo, or oak leaves.
Published European patent application 0 236 992 by Farrier et al. describes a carbon fuel element and process for producing the carbon fuel element. The carbon fuel element contains carbon powder, a binder and other additional ingredients, and consists of between 60% and 70% by weight of carbon.
Published European patent application 0 245 732 by White et al. describes a dual burn rate carbonaceous fuel element which utilizes a fast burning segment and a slow burning segment containing carbon materials of varying density.
These heat sources are deficient because they provide unsatisfactory heat transfer to the flavor bed, resulting in an unsatisfactory smoking article, i.e., one which fails to simulate the flavor, feel and number of puffs of a conventional cigarette.
U.S. Pat. No. 5,076,296 commonly assigned herewith, solved this problem by providing a carbonaceous heat source formed from charcoal that maximizes heat transfer to the flavor bed, thereby releasing a flavored aerosol from the flavor bed for inhalation by the smoker, while minimizing the amount of carbon monoxide produced.
However, all conventional carbonaceous heat sources liberate some amount of carbon monoxide gas upon ignition. Moreover, the carbon contained in these heat sources has a relatively high ignition temperature, making ignition of conventional carbonaceous heat sources difficult under normal lighting conditions for a conventional cigarette.
Attempts have been made to produce non-combustible heat sources for smoking articles, in which heat is generated electrically (e.g., Burruss, Jr., U.S. Pat. No. 4,303,083, Burruss U.S. Pat. No. 4,141,369, Gilbert U.S. Pat. No. 3,200,819, McCormick U.S. Pat. No. 2,104,266 and Wyss et al. U.S. Pat. No. 1,771,366). These devices are impractical and none has met with any commercial success.
Attempts have been made to produce pyrophoric materials comprising metal aluminides that will burn in a controlled fashion, thereby allowing their use as a decoy for heat-seeking missiles (e.g., Baldi, U.S. Pat. No. 4,799,979). These devices, however, combust too rapidly and produce too intense a heat to be used as a heat source in a smoking article.
Attempts have been made to produce a combustible, non-carbonaceous heat source.
U.S. Pat. No. 5,040,552 commonly assigned herewith, is directed to a metal carbide heat source which produces tenfold less carbon monoxide than conventional carbon heat sources.
U.S. Pat. No. 5,188,130 commonly assigned herewith, relates to a metal nitride heat source that also produces substantially no carbon monoxide or nitrogen oxides upon combustion.
Co-pending U.S. Pat. No. application Ser. No. 07/556,732, filed on Jul. 20, 1990 (PM-1347), and commonly assigned herewith, is directed to a heat source comprising carbon and metal carbide that also produces substantially no carbon monoxide or nitrogen oxides upon combustion.
Co-pending U.S. Pat. No. application Ser. No. 07/639,241, filed on Jan. 9, 1991 (PM-1336), and commonly assigned herewith, is directed to a metal carbide heat source that also produces substantially no carbon monoxide upon combustion.
U.S. Pat. No. 4,146,934 commonly assigned herewith, is directed to heat sources comprising a mixture of metal carbide, metal nitride and metal which undergo a staged ignition process.
Co-pending U.S. Pat. No. application Ser. No. 07/732,619, filed on Jul. 19, 1991 (PM-1353), and commonly assigned herewith, is directed to a carbonaceous heat source containing a metal species that produces substantially no carbon monoxide upon combustion.
There have been attempts to remove carbon monoxide from gaseous combustion products.
Dale U.S. Pat. No. 4,317,460 describes an oxidation catalyst adsorbed onto a solid support. The catalyst may be located in either a smoking article or in a filter tip.
Leary et al., Journal of Catalysis, 101, pp. 301-13 (1986) refers to a catalyst for the oxidation of carbon monoxide produced by an internal combustion engine. These catalysts, however, are derived from expensive metals.
Haruta et al., Journal of Catalysis, 115, pp. 301-309 (1989) refers to production of an oxidation catalyst for the low-temperature conversion of carbon monoxide.
Walker et al., Journal of Catalysis, 110, pp. 298-309 (1988) refers to an iron oxide-based catalyst for the simultaneous oxidation of carbon monoxide and propane.
Published PCT patent application 90/04930 by Schlatter et al. describes a carbonaceous fuel element having coated on its surface a metal catalyst which reduces carbon monoxide emissions.
These attempts have not produced a composition having all of the advantages of the present invention.
It would be desirable to provide a heat source that liberates substantially no carbon monoxide upon combustion.
It would also be desirable to provide a heat source that has a low temperature of ignition to allow for easy lighting under conditions typical for a conventional cigarette, while at the same time providing sufficient heat to release flavors from a flavor bed.
It would also be desirable to provide a durable heat source produced by simplified manufactured techniques.